para-Xylene is one of the major intermediates of petrochemistry. It is transformed into terephthalic anhydride or acid or into methyl terephthalate, and then subsequently polycondensed with diethylene glycol, for example. The polyester PET obtained is then converted into synthetic fibres or resins intended especially for the textile industry, for packaging drinks, and the like.
para-Xylene is generally separated from the other C8 aromatic isomers either by crystallization or by industrial chromatography also known as separation by simulated moving bed adsorption. The technique of separation by crystallization today represents less than 20% of the total production of para-xylene, whereas separation by simulated moving bed adsorption today represents about 80% to about 85% of this production of para-xylene. Finally, about 2% of the total production of para-xylene originates from the combination of the two abovementioned technologies, in which crystallization is used as the finishing step.
Industrial chromatography separation techniques are performed in liquid phase or in gas phase, but separation by industrial chromatography is most generally performed in liquid phase.
Simulated moving bed separation is understood here in the broad sense, i.e. it may be either a simulated counter-current, or a simulated co-current, or a “Varicol” process. The common feature to this family of processes is that the solid adsorbent is used in a fixed bed, and the flows, which are liquid or optionally gaseous in contact with the adsorbent, are governed either by means of a set of on-off valves, or by means of a complex single valve known as a rotary valve.
When the active element of the solid adsorbents used as adsorption agents in these processes is a zeolite, it is obtained in the form of powder (crystals), and preferably used at the industrial scale in the form of agglomerates. These zeolite adsorbents, agglomerated in the form of platelets, beads or extrudates, generally consist of a zeolite powder, which constitutes the active element as regards the adsorption and of a binder intended to ensure the cohesion of the crystals in the form of beads or extrudates, generally known as grains.
This binder also gives the grains sufficient mechanical strength to withstand the mechanical stresses to which they are subjected during their use in units. These mechanical stresses are the cause of the formation of fines, which bring about a deterioration in performance during the operating time of the process.
The processes for separating xylenes in a simulated moving bed (SMB) have undergone numerous technological improvements in recent decades, especially as regards the fluid distribution plateaux, but relatively few changes regarding the particle size characteristics of the solid adsorbent.
The prior art documents describing the chemical and microscopic characteristics of the zeolite adsorbents used for separation of para-xylene are particularly abundant, and mention may be made, for example, by way of illustration, of patents U.S. Pat. Nos. 3,558,730, 3,663,638, 3,960,774, 7,820,869, 7,812,208, 8,283,274, 8,530,367 and 8,735,643.
The general teaching regarding the chemical characteristics of these solid adsorbents is that it is necessary to use a zeolite of faujasite structure (zeolite LSX, X or Y) and in which the compensating ions are in major amount Ba2+ ions or in major amount Ba2+ ions and in minor amount K+ ions.
In addition, the general teaching regarding the microscopic characteristics of the adsorbent is that zeolite X crystals preferably less than 1.6 μm in size (number-average diameter) may be used. Some of the most recent documents teach the use of adsorbents based on zeolite X crystals less than 0.5 μm in size (US 2009/326 308) or between 0.1 μm and 0.4 μm in size (CN1267185C), in order to improve the performance of the process for separating xylene isomers due to a gain in material transfer of these adsorbents compared with the conventional adsorbents mentioned above.
Moreover, in the field of catalysis, especially in hydrocracking processes, when zeolite supports are used, it is common practice to seek to improve the accessibility of the molecules to the micropores of said zeolites by creating mesopores in the zeolite FAU (zeolite Y) crystals by post-synthesis treatment.
The studies by Inayat et al. (Angew. Chem. Int. Ed., (2012), 51, 1962-1965) teach that it is possible to synthesize mesoporous zeolite X crystals. It may thus be expected that the accessibility to the micropores is improved relative to that of conventional X crystals. Consequently, a person skilled in the art would be inclined to wish to use such mesoporous zeolite X crystals to form efficient adsorbents for the separation of xylene isomers.
However, this publication (Inayat, ibid.) shows a 23% loss of the micropore volume of the synthesized mesoporous zeolite NaX, relative to that of conventional zeolite NaX. This is in total contradiction with a selective adsorption volume that is as large as possible, which is desired for the adsorbents used in the separation of xylene isomers.
The observation cannot be avoided that, between the hypothetical gain in material transfer and the confirmed loss of micropore volume, it is impossible to predict the performance of an adsorbent prepared with mezoporous zeolite X crystals for the separation of xylene isomers.
On the other hand, the general teaching concerning the macroscopic characteristics of the adsorbent is that the content of active material can be increased, by transforming the binder into zeolite under the action of a basic alkaline solution, such that the finished product contains a reduced amount of non-zeolitic phase, which may be quantified by reference to an adsorbent composed solely of zeolite, in powder form, from adsorption measurements or from XRD peak intensities. This transformation of the binder into active material for the purposes of adsorption moreover makes it possible to maintain the mechanical strength of the agglomerate (U.S. Pat. No. 8,530,367), which is necessary for withstanding the mechanical stresses during their use in units.
The prior art documents precisely describing the granulometric and morphological characteristics of zeolite adsorbents in association with the distribution plateaux technology of the simulated moving bed process are much rarer and markedly less precise. At the present time, a person skilled in the art does not appear especially to have available any document teaching how to select the optimum particle size characteristics of adsorbents as a function of the properties of the zeolite X crystals used to form the adsorbent.
Some of the documents mentioned previously (for example U.S. Pat. Nos. 3,960,774, 7,812,208, US 2009/326 308) mention the usual agglomeration techniques (extrusion optionally followed by crushing, agglomeration in a frustoconical mixer, for example Nautamix®, in a granulating drum, spheronizator) for obtaining adsorbents in the form of extrudates, spheres or beads in the particle size category ranging from 0.25 mm to 1.2 mm (16-60 standard US mesh size), irrespective of the diameter of the zeolite X crystals used in the adsorbent. In Example 1 of US 2011/105 301, the authors disclose adsorbents in agglomerated form, obtained by agglomeration in a granulating drum (“tumbling”) followed by screening in the particle size category ranging from 0.35 mm to 0.8 mm.
In these documents, the numerical indications regarding the particle sizes of the adsorbents correspond in practice to the mesh aperture of the two gauzes used for selecting the agglomerates, i.e. they correspond to the lower and upper limits of the smallest and largest agglomerate of the distribution, but do not mention mean diameter values.
Patent CN1267185 stresses the importance of a narrow particle size distribution for improving the filling of adsorbents in industrial units: it discloses an adsorbent based on X crystals between 0.1 μm and 0.4 μm in size, in the form of particles with a diameter of between 0.35 mm and 0.8 mm, and satisfying the following distribution: 20% by weight to 25% by weight in the particle size category 0.60 mm to 0.80 mm, 50% by weight to 60% by weight in the particle size category 0.46 mm to 0.60 mm and 20% by weight to 30% by weight in the particle size category 0.35 mm to 0.46 mm.
Given that the particle size distributions of the agglomerates obtained via the standard forming techniques generally follow a lognormal law, a particle size distributed between 0.25 mm and 1.2 mm (for instance in U.S. Pat. No. 7,820,869 and US 2009/326 308) or between 0.35 mm and 0.8 mm (for example in US 2011/105 301) corresponds to a weight-average diameter for a distribution derived from screening of the agglomerates, which is in the region of the median values, namely at about from 0.55 mm to 0.65 mm, as a function of the standard deviation of the distribution.
U.S. Pat. No. 8,530,367 discloses only the number-average diameters of the adsorbents in the form of beads or extrudates, obtained by extrusion, compacting or agglomeration, namely a number-average diameter ranging from 0.4 mm to 2 mm and in particular from 0.4 mm to 0.8 mm, but no mention is made of the lower and upper limits of the smallest and largest agglomerate of the distribution. It is simply stated that the finest agglomerated particles may be removed by cycloning and/or screening and/or the particles that are too coarse by screening or crushing, in the case of extrudates, for example.
It should be noted that a number-average diameter of 0.4 mm corresponds to a higher volume-average diameter, typically from 0.45 mm to 0.55 mm depending on the standard deviation of the distribution, and similarly a number-average diameter of 0.8 mm would correspond to a volume-average diameter of about 0.9 mm to 1.0 mm. The examples of U.S. Pat. Nos. 7,452,840 and 8,530,367 disclose agglomerates with an equivalent diameter equal to 0.7 mm obtained by extrusion, which are subjected to crushing and screening, both for the agglomerates prepared from conventional zeolite X crystals of 2.1 μm and for the agglomerates prepared from zeolite X crystals with a diameter reduced to 1.6 μm.
Consequently, the prior art shows that, irrespective of the properties of the zeolite X crystals used in the adsorbents, the particle size of the adsorbents remains unchanged.
If reference is made to the theoretical expression of the resistance to material transfer as described by Ruthven in Principles of Adsorption and Adsorption Processes, p. 243, the size of the adsorbents is a parameter that it would optionally be sought to reduce in order to promote the material transfer since the diffusional resistance between the crystals (also known as the “macroporous resistance”) is proportional to the square of the diameter of the adsorbents. However, the reduction of the size of the adsorbents is limited by the impact that this would then have during the use of the adsorbent in the industrial application, since the particle size of the adsorbents determines the pressure loss in the industrial unit and the uniformity of packing.
Moreover, it is known in the literature relating to concrete granulates that, for mixtures consisting of irregular-shaped particles having broad particle size distributions, the compactness reduces if the shape of the particles departs from sphericity. Moreover, the compactness increases with the spreading of the particle size distribution since the small particles can become lodged in the interstices created between the larger particles (Cumberland and Crawford, The Packing of particles, Elsevier, Amsterdam, (1987); German, Introduction to particle packing in Powder packing characteristics, (1989) pp. 1-20).
The compactness of the adsorbent bed, which it is desired to maximize in order to achieve maximum productivity with a given adsorbent, also depends on the mode of packing. The filling of the adsorbent in industrial units must be done as densely as possible so as to reduce as much as possible the void fraction left between the beads (Bed porosity: εb).
The difference between “loose” loading without taking particular care and “dense” loading is easily up to 10% of the bulk packing density (grams of adsorbent per m3 of bed). A method of choice for performing dense loading consists in creating a homogeneous “rain” of adsorbent over the entire surface of the bed and in leaving the level to rise sufficiently slowly (several hours). Various commercial devices (Densicat®, Catapac®, Calipac®) exist for doing this. The principle consists in resuming a vertical flow at a controlled rate of adsorbent via a series of horizontal straps or wheels in rotation (at a precisely determined angular speed) so as to spray the adsorbent from the centre to the periphery.
Patent application WO 2008/152 319 describes agglomerates of controlled size and morphology and with very high sphericity obtained via a particularly advantageous forming technique, namely atomization. Said document shows that it is possible to obtain by atomization beads that are both sufficiently dense and mechanically strong and which have very good sphericity in a size range (mean diameter) from 50 μm to 600 μm and preferably from 50 μm to 500 μm. However, said document does not teach how to select the optimum characteristics (especially size and morphology) of the adsorbent as a function of the properties of the zeolite crystals.
It is moreover an ongoing aim to seek constantly to improve the production efficiency of the separation of para-xylene from aromatic fractions containing 8 carbon atoms. To achieve this aim, the abovementioned prior art teaches that one solution might be to improve the intragranular material transfer within the adsorbent, and to increase the amount of adsorbent per bed volume and to decrease the porosity of the bed, i.e. to have available a more compact and denser bed of adsorbent.
According to the theoretical expression of the material transfer resistance as described by Ruthven in Principles of Adsorption and Adsorption Processes, p. 243, one of the solutions for achieving this aim would also be to decrease the size of the zeolite adsorbent beads. However, one of the drawbacks directly linked to the ever greater reduction in size of the beads is a consequent increase in the loss of pressure. To compensate for the increase in the loss of pressure, it is necessary to increase the pressure that must be applied to the adsorbent beds and thus take the risk of breaking the zeolite adsorbents in the beds.
Thus, the prior art more preferentially recommends the solution which consists in further reducing the size of the zeolite crystals within zeolite adsorbents whose bead size (volume-average diameter) is not less than the value of the order of 0.5 mm.
There thus remains a need for zeolite adsorbents in the form of agglomerates comprising crystals of zeolite FAU of X type, in which the compensating ions are predominantly barium ions or barium and potassium ions, making it possible to further increase the production efficiency of para-xylene, without increasing the pressure loss in units for separating para-xylene in a simulated moving bed.
The term “predominantly” used above to characterize the compensating ions present in the type X zeolite means a molar amount of greater than 50% of ions relative to the total molar amount of ions present in the cationic sites of said type X zeolites. It should be understood that all the cationic sites of said zeolites X are occupied by a charged compensating cation, the zeolites used being electronically neutral during their use.
The need for even more efficient zeolite adsorbents in applications for the separation of para-xylene in a simulated moving bed described above may also be expressed by a need for zeolite adsorbents that would combine the following properties:                selective adsorption volume of the zeolite adsorbent that is as large as possible per unit bed volume, i.e.:                    α) the largest possible zeolite content, zeolite constituting the microporosity within which the selective adsorption takes place;            β) the smallest possible grain porosity (high grain density)            γ) the smallest possible porosity of the adsorbent bed (high compactness)                        the fastest possible material transfer within the zeolite adsorbent, i.e. a minimum time for a hydrocarbon molecule to go from the exterior of the adsorbent to the core of the zeolite crystals of the zeolite adsorbent.        
In other words, there is still a need for zeolite adsorbents that combine both optimum micropore transfer and improved macropore transfer while at the same time conserving a sufficient selective adsorption volume per unit bed volume so as to maximize the gain in productivity during their use in processes for separating para-xylene in separation units using the simulated moving bed technique.